cvgmmartin

Thanks for those suggestions. The flat out in 5th test will have to wait for the snow to clear and the first trial since the bike is not licenced for the road and I live in the city. Is there any physical measurement of float position relative to carb body that can be checked to make sure I am in the right ballpark?

I have just finished rebuilding my 1976 325 Sherpa after many years of storage. Way back I converted it to a Mikuni carb as was the fashion years ago but now am going back to the Bing unit based on the comments I have read on other posted topics. The bike was not run for long with the Bing and the slide is not loose in the body so I think it is in reasonable condition. I have it running and it idles nicely but when opening the throttle from idle the power comes in quicker than I would like, I would expect it to be a smoother transition as the throttle is opened. The air screw is one turn out, the needle is in the middle position, and the timing is between 2.4 and 2.6mm BTDC. Not having access to other Bultacos around here I don't know if this is typical of a Bing or not. Does anyone have any suggestions on tuning to smooth power from idle.

I new Charlie Reynolds and I am sure there will be some interesting bikes in his collection, maybe even his old Zundapp trials bike?

There have been many questions over the years concerning carburation problems on 4 strokes (mainly cubs and B 40s) in which riders complain of a flat spot or hesitation on opening the throttle relatively quickly from idle. There seem to be two approaches to tuning a 4 stroke, namely fit the engine with soft cams and a high compression piston or with sporty cams and a low compression piston. I wonder to what extent the perceived carburation problem is also a function of valve timing. I would expect the sporty cams would be more difficult to setup at the low end and perhaps aggravate the problem rather than the soft cam approach. Does anyone have any opinion or direct experience with the two? Do Serco recommend the soft or sporty cam setup? Also the modern 4 strokes, Montesa etc seem to use over square engines with relatively high compressions, does anyone know what valve timing is used on these bikes and how much advance and retard is built into their ignition systems? P.S. I have no intention of building a 12:1 compression B40, I am just curious.

As a begginer completing a "difficult" trial even if it means getting fives most of the time is an achievement to be proud of, and even more so if you managed to enjoy yourself, but it is hard to get the sort of practice needed to improve if you can't make a reasonable attempt at the section. Ride these trials by all means because it gives you an idea what to aim for, but ride in some begginer trials where you are able to get through sections with a few cleans and single scores, that way you get to learn from the successes and not from trying to figure out every time what went wrong. Like Max1956, when I started riding in the late 60s there was only one line and riding a 250cc James commando (approx 300lb, not the villiers engined one) around a midland centre trial with sections laid out for the likes of Scott Ellis, Jeff Smith, Sam and Bob Cooper, Steve Wilson, etc was not a good learning experience and infact put me off trials. I took up grass track racing instead and only came back to trials (vintage that is) after moving to Canada.

Hi Smiley, thanks for your comment. I am now up the tree having made a sprocket and used it in a trial a couple of weeks ago. It seems to work fine and is no problem starting since I use a valve lifter. I would prefer to lower the gearing within the gearbox as you suggest since the current gear spacings are not great. I didn't know there were lower gears available that would fit the B25T cases. You suggest using C15S bottom gear but the Roy Bacon book I have on BSA singles says that the C15S bottom gear is 2.667:1 while the B25T is 2.652:1. This wouldn't change things much, am I missing something? your help would be appreciated. Also there are virtually no C15 bikes on this side of the pond so can you suggest a possible source other than E Bay?

You should consider putting a 500 triumph engine in it, the twin is a much shorter engine than a single so is easier to fit in a two stroke frame. I put one in a Bultaco sherpa and the only frame change was to cut out the bottom tubes and make a alloy sump plate which then bolted onto the front down tube and onto the frame tubes just below the swinging arm pivot. An oil tank was made to fit under the seat where the air filter normally goes. The engine is also easy to tune or detune depending on use. The only problem with this special was that it didn't steer very well, it looked good but it was lousy to ride, not until a made a special frame and was able to play with weight distribution and steering head angles and trail was I able to get it to ride and steer like a trials bike should. I remember seeing in the Motorcycle news many years ago (more than 30 years) a special with a 500 triumph engine in a Yamaha enduro chassis, the perfect green laner.

I have put a Keihin CV carb with an accelerator pump on my 500 triumph trials bike (I ride in Canadian vintage trials) and it works fine, it idles very well with very clean pick up, probably due to the accelerator pump more than the CV design. It will easily give me instant power to get over large logs (eg 3ft plus). Stiffening up the diaphragm spring very slightly improved it a little and the plug colour is good throughout the range.

The normal engine sprocket for trials use is 18 teeth and yes you can remove some links if the chain is too long. If you don't have the device for breaking chains then you can grind back the rivetted end of the pins on one link and knock them out with a punch. If your current chain is does not have a joining link then you will need to buy one.

Your math is correct Matchless man, the 15 tooth allows me to reduce the rear wheel from 60 to 50 teeth (60 teeth looks ugly). The gear box has a 12 tooth sprocket which I could change to 11 but that won't help much. In Canada (i.e. Ontario) the vintage trials sections are fairly tight since the bikes are nearly all more modern twin shocks with several people riding monoshocks, hence the need to lower the gearing probably more than would typically be done in the UK. You would think that with a country this large that they could design a few long sections, but no. I normally ride a 500 triumph and figured I would try a "light weight' 250 BSA. I just finished making a 15 tooth engine sprocket and tried it out at a recent trial and it seems O.K. when used in conjunction with the 50 tooth on the rear wheel. I made it by grinding off the teeth from an old sprocket to form a hub and then welding on t "weld on" sprockets, the end result wasn't too bad. Will a WD B40 set of gears fit in the B25T cases? and where on earth could I get a set?, it would be a preferable solution.

Thanks, that seems to work. I did get a reply from Burton Bike Bits, unfortunately their answer was that only an 18 tooth was available. Looks like I will be making one. I may have to invest in some more tools, the current collection of just a drill, hacksaw, grinder, welder, and files is making bike building too ,more challenging than it need be.

Thanks for the suggestion. I have sent two e-mails to them but they don't seem to be getting through. I will keep trying, maybe their system is down for some reason

Thanks for the suggestion, unfortunately they only have the 18 tooth sprocket. The more I talk to people about this question the more I believe it hasn't been done before. Hopefully someone can prove me wrong

Can anyone tell me where I can get a 15 tooth engine sprocket for my B25T engine. I believe it would have the same spline size as for a B40 or 441 BSA. I believe most people use an 18 tooth sprocket (which I currently have) but I know that with a half link in the duplex chain a 15 tooth sprocket will fit without the chain being too loose. Although not as strong as a full link, does anyone know of any problem using half links? I have already reduced the gearbox sprocket as small as I can and even with a 60 tooth rear wheel sprocket the gearing is barely low enough, so since I can't lower the gearing within the gearbox I prefer to get a smaller engine rather than put a large rear sprocket on the back wheel.

I ride a 500 Triumph twin in Canadian vintage trials but since nobody else around here rides one I have had to fumble through design mods and improvements myself which in turn has usually meant doing it myself at next to no cost. Also since our trials have fairly tight sections, the engine has to have really good low speed running performance. As far as the engine is concerned, I have found that using standard cams (E3134 inlet) and rotating the inlet cam gear relative to the idler gear such that the valve opens around 30 deg BTDC (I think it was 2 teeth, but easy to check), the low speed running and pick up was better and it was less prone to stalling. I left the exhaust valve timing as standard. Quick response from idle comes from an old Keihin carb with accelerator pump, it no longer coughs and stalls when quickly opening the throttle after a long descent. My electronic ignition system using an ignition box from a Yamaha twin and a pick up coil that I have made to fit where the contact breakers are normally mounted. I use just a 12v battery without charging system since our trials are multiple laps and long distance rides are not an issue and this has left space for an external flywheel to be put on the end of the crankshaft where the alternator normally is. This gives a marginal improvement in bottom end power and seems to give more grip in the mud. I made the frame myself and found that handling was very sensitive to the fork angle and trail. I probably cut the steering head off atleast 5 times before getting it right. A little front axle lead of 3/4 inch (i.e. axle infront of fork centreline helped keep the steering light but too much caused the wheel to turn in on itself on down hill turns. The wheel base is 51 1/2 inches similar to modern bikes so it turns as well as a TY 250. Wheels, forks, and shocks come from old Honda/Yamaha trail bikes which keep costs down (sometimes even free). Instead of buying expensive rear shocks I have found that for slow sections over rocks and trees it rides well with no damping, the weight of the bike probably provides sufficient damping effect. Worn out shocks are not collector's items so you should be able to get them fairly cheap. It probably wouldn't be so good for a flat out in third hill climb over rough ground but then unfortunately we don't have any sections like that. Although it wouldn't qualify as a pre 65 bike, I don't feel guilty since I have to ride it against Yamahas, Bultacos, Shercos, Fantics, and Hondas. Over the years I have probably only spent about 1000 dollars on it (500 pounds). I am shocked at the high prices of some of the bikes and parts in the UK and I would hate to think of the cost if I had to buy parts from the UK and shipped them over here. I did once try to buy some 3TA cams once from a well known southern UK dealer (not Sammy) and I was mailed a pair of cams that must have been sitting on a wet metal shelf and were badly corroded and obviously useless, so I get nervous ordering parts from overseas and prefer to do/make things myself.

I had a similar syptoms on my Bultaco, in my case the cure was replacing the crankshaft seal. Are you using a plain plug cap?

Hi Charlie, I am new to this site so my response may be a little late, but in answer to your posting, the trial was the "Good Companions Trial" organized by the Leamington Victory Motorcycle Club and the trial was usually held in mid January. I last rode in that trial when I was about 18yrs old and rode a B40 engine in a Firefly frame (the one by John Lee). Although I didn't see Ron Langston go up King John's Lane I did see Scott Ellis, Jeff Smith, and Dave Langston on their works BSAs and Ray Round on his 441 sidecar outfit go up the hill. Scott was flat out in third but Jeff was a little more restained, lesser creatures legged their way up. This was almost the last section of the trial and was meant to provide spectator entertainment. There were always crowds on the banks either side of the lane. If you couldn't get up it was a very long ride out at the bottom and round the roads back to the quarry. I notice you are from Banbury, I rode in a few Banbury club trials , the last being at Shennington. I think John Gleed was riding his B40. One year (1969 I think) I won the Banbury club's Big Banger trophy since Frank Night didn't ride his 500 Triumph that year and I was the only one with a bike over 250cc. I was 18 and weighed about 160 lbs, the B40 was way too heavy for me and seemed to get an awful lot of fives. I now ride vintage trials in Canada with a 500 Triumph and a 250 BSA. We don't have many true vintage bikes over here so we allow anyone to ride in our events. Typically we get about 20 riders with a mixture of monoshocks, twin shock TY Yamahas, a couple of cubs, and a 500 Triumph. We certainly don't have the trick cubs /BSAs and I have never even seen a B40 over here.